1. Field of the Invention
The present invention relates to the recovery of metal values from refractory sulfide and refractory carbonaceous sulfide ores.
2. Description of the Prior Art
Gold is one of the rarest metals on earth. Gold ores can be categorized into two types: free milling and refractory. Free milling ores are those that can be processed by simple gravity techniques or direct cyanidation. Refractory ores, on the other hand, are not amenable to conventional cyanidation treatment. Such ores are often refractory because of their excessive content of metallic sulfides (e.g., pyrite) and/or organic carbonaceous matter.
A large number of refractory ores consist of ores with a precious metal such as gold occluded in iron sulfide particles. The iron sulfide particles consist principally of pyrite and arsenopyrite. Precious metal values are frequently occluded within the sulfide mineral. For example, gold often occurs as finely disseminated sub-microscopic particles within a refractory sulfide host of pyrite or arsenopyrite. If the gold remains occluded within the sulfide host, even after grinding, then the sulfides must be oxidized to liberate the encapsulated precious metal values and make them amenable to a leaching agent (or lixiviant).
A number of processes for oxidizing the sulfide minerals to liberate the precious metal values are well known in the art. One known method of oxidizing the metal sulfides in the ore is to use bacteria, such as Thiobacillus ferrooxidans, Sulfolobus, Acidianus species and facultative-thermophilic bacteria in a microbial pretreatment. The foregoing microorganisms oxidize the iron sulfide particles to cause the solubilization of iron as ferric iron, and sulfide, as sulfate ion.
If the refractory ore being processed is a carbonaceous sulfide ore, then additional process steps may be required following microbial pretreatment to prevent preg-robbing of the aurocyanide complex or other precious metal-lixiviant complexes by the native carbonaceous matter upon treatment with a lixiviant.
As used herein, sulfide ore or refractory sulfide ore will be understood to also encompass refractory carbonaceous sulfide ores.
A known method of bioleaching carbonaceous sulfide ores is disclosed in U.S. Pat. No. 4,729,788, issued Mar. 8, 1988, which is hereby incorporated by reference. According to the disclosed process, thermophilic bacteria, such as Sulfolobus and facultative-thermophilic bacteria, are used to oxidize the sulfide constituents of the ore. The bioleached ore is then treated with a blanking agent to inhibit the preg-robbing propensity of the carbonaceous component of the ore. The precious metals are then extracted from the ore using a conventional lixiviant of cyanide or thiourea.
Another known method of bioleaching carbonaceous sulfide ores is disclosed in U.S. Pat. No. 5,127,942, issued Jul. 7, 1992, which is hereby incorporated by reference. According to this method, the ore is subjected to an oxidative bioleach to oxidize the sulfide component of the ore and liberate the precious metal values. The ore is then inoculated with a bacterial consortium in the presence of nutrients therefor to promote the growth of the bacterial consortium, the bacterial consortium being characterized by the property of deactivating the preg-robbing propensity of the carbonaceous matter in the ore. In other words, the bacterial consortium functions as a biological blanking agent. Following treatment with the microbial consortium capable of deactivating the precious-metal-adsorbing carbon, the ore is then leached with an appropriate lixiviant to cause the dissolution of the precious metal in the ore.
Problems exist, however, with employing bioleaching processes in a heap leaching environment. These include nutrient access, air access, and carbon dioxide access for making the process more efficient and thus an attractive treatment option. Moreover, for biooxidation, the induction times concerning biooxidants, the growth cycles, viability of the bacteria and the like are important considerations because the variables such as accessibility, particle size, settling, compaction and the like are economically irreversible once a heap has been constructed. As a result, heaps cannot be repaired once formed, except on a limited basis.
Ores that have a high clay and/or fines content are especially problematic when processing in a heap leaching or heap biooxidation process. The reason for this is that the clay and/or fines can migrate through the heap and plug channels of air and liquid flow, resulting in puddling; channelling; nutrient-, carbon dioxide-, or oxygen-starving; uneven biooxidant distribution, and the like. As a result, large areas of the heap may be blinded off and ineffectively leached. This is a common problem in cyanide leaching and has lead to processes of particle agglomeration with cement for high pH cyanide leaching and with polymers for low pH bioleaching. Polymer agglomerate aids may also be used in high pH environments, which are customarily used for leaching the precious metals, following oxidative bioleaching of the iron sulfides in the ore.
Biooxidation of refractory sulfide ores is especially sensitive to blocked percolation channels by loose clay and fine material because the bacteria need large amounts of air or oxygen to grow and biooxidize the iron sulfide particles in the ore. Air flow is also important to dissipate heat generated by the exothermic biooxidation reaction, because excessive heat can kill the growing bacteria in a large, poorly ventilated heap.
Ores that are low in sulfide or pyrite, or ores that are high in acid consuming materials such as calcium carbonate or other carbonates, may also be problematic when processing in a heap biooxidation. The reason for this is that the acid generated by these low pyrite ores is insufficient to maintain a low pH and high iron concentrate needed for bacteria growth.
A need exists, therefore, for a heap bioleaching technique that can be used to biooxidize precious metal bearing refractory sulfide ores and which provides improved air and fluid flow within the heap. In addition, a need exists for a heap bioleaching process in which ores that are low in sulfide minerals, or ores that are high in acid consuming materials such as calcium carbonate, may be processed.
A need also exists for a biooxidation process which can be used to liberate occluded precious metals in concentrates of refractory sulfide minerals. Mill processes that can be used for oxidizing such concentrates include bioleaching in a stirred bioreactor, pressure oxidation in an autoclave, and roasting. These mill processes oxidize the sulfide minerals in the concentrate relatively quickly, thereby liberating the entrapped precious metals. However, unless the concentrate has a high concentration of gold, it does not economically justify the capital expense or high operating costs associated with these processes. And, while a mill bioleaching process is the least expensive mill process in terms of both the initial capital costs and its operating costs, it still does not justify processing concentrates having less than about 0.5 oz. of gold per ton of concentrate, which typically requires an ore having a concentration greater than about 0.07 oz. of gold per ton. Therefore, a need also exists for a process that can be used to biooxidize concentrates of precious metal bearing refractory sulfide minerals at a rate comparable to a stirred tank bioreactor, but that has capital and operating costs more comparable to that of a heap bioleaching process.
In addition to concentrates of precious metal bearing sulfide minerals, there are many sulfide ores that contain metal sulfide minerals that can potentially be treated using a biooxidation process. For example, many copper ores contain copper sulfide minerals. Other examples include zinc ores, nickel ores, and uranium ores. Biooxidation could be used to cause the dissolution of metal values such as copper, zinc, nickel and uranium from concentrates of these ores. The dissolved metal values could then be recovered using known solvent extraction techniques, iron cementation, and precipitation. However, due to the sheer volume of the sulfide concentrate formed from sulfide ores, a stirred bioreactor would be prohibitively expensive, and standard heap operations would simply take too long to make it economically feasible to recover the desired metal values. A need also exists, therefore, for an economical process for biooxidizing concentrates of metal sulfide minerals produced from sulfide ores to thereby cause the dissolution of the metal values so that they may be subsequently recovered from the bioleachate solution.
It is an object of one aspect of the present invention to provide a heap bioleaching process of the type described above, wherein the refractory sulfide ore is rendered more susceptible to biooxidation, thereby providing improved recovery of the precious metal values contained within the ore. The method of the present invention achieves this object by removing the clays and/or fines from the refractory sulfide ore after it is crushed to a size appropriate for a heap leaching process. The heap may then be formed without concern of the air and liquid flow channels in the heap becoming plugged. Further, if the separated clay and/or fine material has a sufficiently high precious metal content, it may be separately treated to recover the precious metal values contained therein.
In another aspect of the present invention, a process for recovering precious metal values from concentrates of precious metal bearing refractory sulfide minerals is provided. The process comprises the steps of (a) distributing a concentrate of refractory sulfide minerals on top of a heap of support material; (b.) biooxidizing the concentrate of refractory sulfide minerals; (c.) leaching precious metal values from the biooxidized refractory sulfide minerals with a lixiviant; and (d.) recovering precious metal values from the lixiviant. An advantage of this process is that the rate at which the sulfide minerals biooxidize is much higher than would be observed in a traditional heap bioleaching operation. Despite this high rate of biooxidation, however, the initial capital costs and operating costs for the disclosed process are lower than that associated with a mill type biooxidation process.
Gold is the preferred precious metal recovered using the process according to the present aspect of the invention. However, other precious metals can also be recovered, including silver and platinum. The support material is preferably selected from the group consisting of lava rock, gravel, and coarsely ground ore. Lava rock is particularly preferred due to its high surface area. As those skilled in the art will immediately recognize, a number of lixiviants can be used in conjunction with the present process, however, thiourea and cyanide are the preferred, cyanide being a particularly preferred lixiviant.
In another aspect of the present invention a process is provided for recovering metal values from sulfide ores. Such ores include, by way of example, chalcopyrite, sphalorite, nickel sulfide ores, and uranium sulfide ores. The process according to this aspect of the invention comprises (a.) forming a concentrate of metal sulfide minerals; (b.) spreading the concentrate on top of a heap of support material; (c.) biooxidizing the concentrate; and (d.) recovering metal values from the solution used to biooxidize the metal sulfide minerals. Due to the fact that this process uses a heap of support material for the bioreactor, its capital and operating costs are less than that of a mill bioleaching operation. However, due to the good air flow in the heap, the biooxidation rate of the sulfide minerals is quite high and can approach that of what would be observed in a mill type operation.
Depending on the sulfide ore from which the concentrate is obtained, the metal values that can be recovered from the process according to the present aspect of the invention include copper, zinc, nickel and uranium. The support material used in the present process is preferably selected from the group consisting of lava rock, gravel, and coarsely ground rock. Lava rock is particularly preferred due to its high surface area.
The above and other objects, features and advantages will become apparent to those skilled in the art from the following description of the preferred embodiments.